Secreted protein acidic and rich in cysteine (also known as osteonectin, BM40, or SPARC) (hereafter “SPARC”), is a matrix-associated protein that elicits changes in cell shape, inhibits cell-cycle progression, and influences the synthesis of extracellular matrix (Bradshaw et al., Proc. Nat. Acad. Sci. USA 100: 6045-6050 (2003)). The murine SPARC gene was cloned in 1986 (Mason et al., EMBO J. 5: 1465-1472 (1986)) and a full-length human SPARC cDNA was cloned and sequenced in 1987 (Swaroop et al., Genomics 2: 37-47 (1988)). SPARC expression is developmentally regulated, and is predominantly expressed in tissues undergoing remodeling during normal development or in response to injury. For example, high levels of SPARC protein are expressed in developing bones and teeth (see, e.g., Lane et al., FASEB J., 8, 163 173 (1994); Yan & Sage, J. Histochem. Cytochem. 47:1495-1505 (1999)).
SPARC is upregulated in several aggressive cancers, but is absent in the corresponding inormal tissues (Porter et al., J. Histochem. Cytochem., 43, 791 (1995)). SPARC expression is induced among a variety of tumors (e.g., bladder, liver, ovary, kidney, gut, and breast). In bladder cancer, for example, SPARC expression has been associated with advanced carcinoma. Invasive bladder tumors of stage T2 or greater have been shown to express higher levels of SPARC relative to bladder tumors of stage T1 (or less superficial tumors), and poorer prognosis (see, e.g., Yamanaka et al., J. Urology, 166, 2495 2499 (2001)). In meningiomas, SPARC expression has been associated only with invasive tumors (see, e.g., Rempel et al., Clincal Cancer Res., 5, 237 241 (1999)). SPARC expression also has been detected in 74.5% of in situ invasive breast carcinoma lesions (see, e.g., Bellahcene, et al., Am. J. Pathol., 146, 95 100 (1995)), and 54.2% of infiltrating ductal carcinoma of the breast (see, e.g., Kim et al., J. Korean Med. Sci., 13, 652 657 (1998)). SPARC expression also has been associated with frequent microcalcification in breast cancer (see, e.g., Bellahcene et al., supra), suggesting that SPARC expression may be responsible for the affinity of breast metastases for the bone.
Surprisingly, SPARC has also been shown to have anti-tumor activity in some systems. SPARC is a potent cell cycle inhibitor that arrests cells in mid-G1 (Yan & Sage, J. Histochem. Cytochem. 47:1495-1505 (1999)) and the inducible expression of SPARC has been shown to inhibit breast cancer cell proliferation in an in vitro model system (Dhanesuan et al., Breast Cancer Res. Treat. 75:73-85 (2002)). Similarly, exogenous SPARC can reduce the proliferation of both HOSE (human ovarian surface epithelial) and ovarian cancer cells in a concentration-dependent manner. In addition, SPARC induces apoptosis in ovarian cancer cells. Further evidence for SPARC receptors present on cells such as ovarian epithelial cells has been report. It has been proposed that the binding of SPARC to its receptor is likely to trigger tissue-specific signaling pathways that mediate its tumor suppressing functions (Yiu et al., Am. J. Pathol. 159:609-622 (2001)). Purified SPARC has also been reported to potently inhibit angiogenesis and significantly impair neuroblastoma tumor growth in an in vivo xenograft model system (Chlenski et al., Cancer Res. 62:7357-7363 (2002)).
SPARC also plays a role in non-neoplastic proliferative diseases. Mesangial cell proliferation is a characteristic feature of many glomerular diseases and often precedes extracellular matrix expansion and glomerulosclerosis. In a model of experimental mesangioproliferative glomerulonephritis, SPARC mRNA was increased 5-fold by day 7 and was identified in the mesangium by in situ hybridization. However, recombinant SPARC or a synthetic SPARC peptide inhibited platelet-derived-growth-factor-induced mesangial cell DNA synthesis in vitro (Pichler et al., Am. J. Pathol. 148(4):1153-67 (1996)). Similarly, while renal enlargement, due to hyperplasia, hypertrophy, and increase inter-cellular matrix, is a characteristic feature of diabetes in humans, kidney SPARC mRNA levels fell in diabetic animals. In addition, the onset of diabetes-related kidney growth is associated with a reduction in SPARC mRNA and protein (Gilbert et al., Kidney Int. 48(4):1216-25 (1995)).
SPARC has been implicated in the pathogenesis of atherosclerotic lesions. Plasma SPARC levels are elevated in patients with coronary artery disease (Masahiko et al., Obesity Res. 9:388-393 (2001)). The proliferation of vascular smooth muscle cells in the arterial intima plays a central role in the pathogenesis of atherosclerosis. SPARC is expressed in vascular smooth muscle cells and macrophages associated with atherosclerotic lesions. In addition, SPARC has been hypothesized to regulate the action of platelet-derived growth factor during vascular injury (Masahiko et al., Obesity Res. 9:388-393 (2001); Raines et al., Proc. Nati. Acad. Sci. USA 89:1281-1285 (1992)). A stimulating effect of SPARC on endothelial PAI-1 production has been reported at the site of vascular injury (Hasselaar et al., J. Biol. Chem. 266:13178-13184 (1991)) and has been postulated to accelerate atherosclerosis (Masahiko et al., Obesity Res. 9:388-393 (2001)).
SPARC has affinity for a wide variety of ligands including cations (e.g., Ca 2+, Cu 2+, Fe 2+), growth factors (e.g., platelet derived growth factor (PDGF), and vascular endothelial growth factor (VEGF)), extracellular matrix (ECM) proteins (e.g., collagen I V and collagen IX, vitronectin, and thrombospondin 1), endothelial cells, platelets, albumin, and hydroxyapaptite (see, e.g., Lane et al., FASEB J., 8, 163 173 (1994); Yan & Sage, J. Histochem. Cytochem. 47:1495-1505 (1999)). SPARC is also known to bind albumin (see, e.g., Schnitzer, J. Biol. Chem., 269, 6072 (1994)).
Antibody therapy is an effective method for controlling disease wherein a specific protein marker can be identified. Examples include Avastin (anti-VEGF antibody), Rituxan (anti-CD20 antibody), and Remicade (anti-TNF antibody). As such, antibody against SPARC represent an important therapeutic agent for treating human and other mammalian tumors, or other proliferative, hyperplastic, remodeling, and inflammatory disorders, that express the SPARC protein.
Accordingly, there is a need for novel forms of SPARC and antibodies reactive with such novel forms of SPARC. The present invention provides for such novel SPARC polypeptides, nucleic acids which encode such novel SPARC polypeptides, and methods of use of such novel SPARC polypeptides and nucleic acids. The invention additionally provides for antibodies against SPARC polypeptides.